Superchargers such as Roots blowers or twin screw devices can be lossy because the engine crankshaft is always connected by a pulley to the input shaft of the supercharger. Even when the supercharger is idle, the pulley set-up draws torque from the engine crankshaft. Additional losses occur when the supercharger lobes continue rotating when boosting is not required, and the boosted air must be bypassed around the engine.
Further, it is desirable to have different drive speeds of the supercharger, but the complexity and footprint required to do so is prohibitive.